The present invention relates to semiconductor integrated circuits and, more particularly, to the fabrication of integrated circuits requiring deep trenches.
A memory cell in an integrated circuit comprises a transistor with an associated capacitor. The capacitor consists of a pair of conductive layers separated by a dielectric material. Information or data is stored in the memory cell in the form of charge accumulated on the capacitor. As the density of integrated circuits with memory cells is increased, the area for the capacitor becomes smaller and the amount of charge it is able to accumulate is reduced. Thus, with less charge to detect, reading the information or data from the memory cell becomes more difficult.
With increasing levels of integration in semiconductor chips or dies in a wafer, such as those with a silicon substrate, greater demands are placed on the method of fabricating the semiconductor chips, such as making smooth, straight-walled deep trenches especially for use a capacitors. The deep trenches are dry etched using HBr, O2, NF3 and SF6 as the plasma gases. However, if the only fluorinated gas is used NF3 as the etching gas, the deep trench is formed with a controlled narrow profile but with zagged sidewalls which may generate crack propagation and cause merging with adjacent trenches. With increased level of integration or decreased ground rules, this cracking problem becomes even more acute. By using SF6 as the only fluorinated gas in etching the trenches, shallow trenches with broad profiles are undesirably obtained but with desirable smooth sidewalls. Again, with decreasing ground rules, broad profiles can cause trench mergers and will not give the required capacitance for trench capacitor application. When NF3 and SF6 are used together, the etching is difficult to control and blowouts, in the substrate being etched, occur most likely due to one gas dominating over the other. Both NF3 and SF6 are known in the prior art and are disclosed in U.S. Pat. No. 5,935,874 as etchant gases for forming deep trenches in a substrate such as silicon. The etchant plasma of this patent with either of these fluorine containing gases mandates the use of a high percentage helium in the plasma. HBr also is known for use as an etchant gas in forming high aspect ratio, deep trenches as disclosed in U.S. Pat. No. 6,127,278 in a multi step process in which HBr and O2 is used in the first step and a fluorine containing gas is added to the second step to increase the etch rate of the substrate. If desired, the patent states that a small amount of a fluorine-containing gas such as NF3, SF6, SiF4, Si2F6 and the like can be added in a very small amount to the etchant gases in the first step. However, the patent does not state that the fluorine-containing gas must be different from the fluorine-containing gas of the second step. The first etching step deposits a passivation layer on the sidewalls of the opening and produces a taper in the sidewalls at the top of the trench. Neither of these patents nor any of the known prior art teaches how to combine NF3 with SF6 to obtain a controlled narrow-profiled deep trench with smooth sidewalls without a blowout of the substrate.
Accordingly, it is object of the present invention to be able to form controlled narrow-profiled deep trench with smooth sidewalls. Another object of the present invention is to use both NF3 and SF6 as etchants for forming a deep trench but without risking a blowout of the substrate. To achieve these and other objects, the present invention comprises etching of the deep trench by alternating these two etchant gases, one without the other. Preferably, the substrate is first etched at one or more mask openings with NF3 without SF6 for a period of time followed by the removal of NF3 and the etching of the trench in the substrate at the one or more mask openings with SF6 without the NF3 for a further period of time. The alternating etching steps are repeated until the desired etch deep of the trench in the substrate is achieved. If desired, the order of the NF3 and SF6 etchant gases can be reversed in etching the trench. Thus, a smooth sidewalled, narrow-profiled trench is formed without any possibility of a blowout in the substrate because NF3 and SF6 are not combined together during etching of the trench.